Dampers and mounts are known which use a hydraulic fluid as the working medium to create damping forces/torques and to control motion, shock, and/or vibration. One special class of these devices are controllable. In particular, controllable mounts and dampers are known which include Electrorheological fluid (ER), Electrophoretic fluid (EP), Magnetorheological fluid (MR), and Hydraulic fluid (Semi-active), etc. Examples of ER-type mounts and dampers may be found in U.S. Pat. No. 4,733,758 to Duclos et al. and U.S. Pat. No. 5,029,677 to Mitsui. Further discussions of ER devices may be found in SAE 881134 entitled "Design of Devices Using Electrorheological Fluids" by T. Duclos. Descriptions of EP-type dampers may be found in U.S. Pat. No. 5,018,606 to J. D. Carlson. Examples of Semi-Active hydraulic dampers and valves may be found in U.S. Pat. No. 3,807,678 to Karnopp et al. and U.S. Pat. No. 5,207,774 to Wolfe et al.
Of particular interest are Magnetorheological (MR) fluid devices (hereinafter MR devices), as they only require small electrical currents (typically several amps or less) and do not present the potential shock hazard that ER devices do, because they operate on much lower voltage (typically 12 volts or less). MR devices employ a controllable Magnetorheological (MR) fluid comprised of small soft-magnetic particles dispersed within a liquid carrier. Typical particles include carbonyl iron, or the like, having various shapes, but which are preferably spherical, and which exhibit mean dimensions of between about 0.1 .mu.m to 500 .mu.m, and more preferably between about 1 .mu.m and 100 .mu.m. The carrier fluids include various known hydraulic oils, and the like. These MR fluids exhibit a thickening behavior (a rheology change), sometimes referred to as an "apparent viscosity change", upon being exposed to a magnetic field of sufficient strength. The higher the magnetic field strength to which the MR fluid is exposed, the higher the differential pressure (flow restriction or damping force) that can be achieved within the particular MR device (ex. MR damper, MR mounting).
Examples of prior art fluids can be found in WO 94/10694, WO 94/10693, and WO 94/10692, the inventions of which are commonly assigned to the assignee of the present invention. In particular, MR fluid devices provide ease of controllability through simple user selected fluctuations in the electrical current supplied to the magnetic field generator (generally a wound-wire coil) in the device. Notably, MR fluids and devices have demonstrated durability yet unobtained with ER devices (which exhibit a change in rheology upon being exposed to "electric" fields). Further, MR devices provide simplicity previously unachieved with controllable Semi-active devices, in that the controllable valves have few or no moving parts.
Descriptions of prior art MR devices can be found in U.S. application Ser. No. 08/304,005 entitled "Magnetorheological Fluid Devices and Process of Controlling Force in Exercise Equipment Utilizing Same", U.S. Ser. No. 08/613,704 entitled "Portable Controllable Fluid Rehabilitation Devices", U.S. Ser. No. 08/674,371 entitled "Controllable Brake", U.S. Ser. No. 08/674,179 entitled "Controllable Vibration Apparatus" and U.S. Pat. Nos. 5,547,049, 5,492,312, 5,398,917, 5,284,330, and 5,277,281, all of which are commonly assigned to the assignee of the present invention. Notably, these MR devices provide user-variable/selectable control forces or torques and describe such devices as MR mounts, MR dampers, and MR brakes.
Notably, the MR fluid valves (hereinafter MR valve) described in the prior art devices such as MR dampers, MR mounts, and the like, lack the ability to tune the output characteristics of the device/apparatus without some "rapid" control thereof. In other words, they require "rapidly switching" electronics in order to provide "asymmetric" damping in the various directions. For example, different damping characteristics in compression and extension are achieved in the prior art by applying one current in the compression direction and then rapidly switching to a second current in extension.